1. Field of the Invention
The present invention relates to a viewfinder optical system, and more particularly to a viewfinder optical system suitable for use in an optical apparatus, such as a video camera, a digital camera or the like, arranged to enable a viewfinder image (object image) formed as an inverted real image by an objective lens to be observed as a non-inverted erecting viewfinder image by utilizing an image inverting unit as appropriately set.
2. Description of Related Art
Heretofore, in viewfinder systems for optical apparatuses, such as photographic cameras and video cameras, there have been proposed a variety of viewfinder optical systems of the real image type in which a real image formed on a primary image forming plane by an objective lens is converted into an erecting image and the erecting image is viewed through an eyepiece lens. Such a viewfinder optical system of the real image type makes it easier to reduce the size of the whole optical system than a viewfinder optical system of the virtual image type. Therefore, these days, the viewfinder optical system of the real image type is widely used in cameras having zoom lenses.
A viewfinder optical system of the real image type using a Porro prism for forming a non-inverted erecting image has such a tendency that a part of the Porro prism protrudes in the height (up-and-down) direction and the width (right-and-left) direction of an optical apparatus according to the external form of the Porro prism, thereby enlarging the whole viewfinder system. In order to shorten the total lens length of a viewfinder optical system according to a decrease in size and thickness of the whole camera, in Japanese Laid-Open Patent Application No. Hei 6-167739 (corresponding to U.S. Pat. No. 5,640,632), there is disclosed a small-sized viewfinder optical system in which an optical path leading to a primary image forming plane on which an object image is formed by an objective lens is bent by a reflecting surface and the primary image forming plane is thus formed inside an image inverting unit.
FIG. 30 is a sectional view showing essential parts of a conventional viewfinder optical system using a prism and a roof prism for bending an optical path leading to a primary image forming plane. In FIG. 30, reference character OL denotes an objective lens, and reference character P denotes a prism for forming a non-inverted erecting image. The prism P is composed of a first prism P1 and a second prism (roof prism) P2. Reference character S denotes a field frame, which is disposed within a narrow space across which an exit surface 13 of the first prism P1 and an entrance surface 21 of the second prism P2 are opposed to each other. A viewfinder image formed as an inverted real image by the objective lens OL is formed in the vicinity of the field frame S through the first prism P1. Reference character EL denotes an eyepiece lens, which is used for observing a non-inverted erecting viewfinder image into which the inverted real viewfinder image formed in the vicinity of the field frame S is converted through the second prism P2.
In the viewfinder optical system shown in FIG. 30, if it is designed to increase an angle of field, it is necessary to enlarge the second prism P2, so that there is a tendency for the size of the camera to increase in the thickness (depth) direction thereof. Meanwhile, in the viewfinder optical system, the focal length xe2x80x9cfexe2x80x9d of the eyepiece lens corresponds to the length from the image forming position to the eyepiece lens. Then, assuming that the focal length of the objective lens is denoted by xe2x80x9cfoxe2x80x9d, the viewfinder magnification xcex3 is expressed by the following equation:
xcex3=fo/fe
Accordingly, if the second prism P2 is enlarged so as to increase an angle of field, an optical path from the image forming position to the eyepiece lens EL becomes longer.
Thus, the focal length xe2x80x9cfexe2x80x9d of the eyepiece lens EL becomes longer to decrease the viewfinder magnification xcex3, so that it becomes difficult to observe a good viewfinder image.
With regard to an image inverting unit which is small in size and is capable of enlarging an angle of view and a viewfinder magnification, for example, in Japanese Laid-Open Patent Application No. Hei 8-179400 and Japanese Laid-Open Patent Application No. Hei 10-206933, there is disclosed a viewfinder optical system in which two prisms are disposed with an air gap put at a minute interval therebetween.
FIGS. 31 and 32 show the basic construction of a viewfinder optical system of the real image type using a prism and a roof prism for bending an optical path leading to a primary image forming plane, which construction is similar to that disclosed in Japanese Laid-Open Patent Application No. Hei 10-206933. In FIGS. 31 and 32, reference character OL denotes an objective lens, and reference character P denotes a prism for forming a non-inverted erecting image. The prism P is composed of a first prism P11 and a second prism (roof prism) P12, and an exit surface 111 of the first prism P1 and an entrance surface 121 of the second prism P2 are disposed, in parallel, with a minute air gap xe2x80x9cdxe2x80x9d put therebetween. Reference character S denotes a field frame, which is disposed in the vicinity of an exit surface 123 of the second prism P2 (on a primary image forming plane). A viewfinder image formed as an inverted real image by the objective lens OL is converted, through a roof reflecting surface 122 of the second prism P12, into a non-inverted erecting image, which is formed in the vicinity of the field frame S. Reference character EL denotes an eyepiece lens, which is used for observing the non-inverted erecting viewfinder image formed in the vicinity of the field frame S through the second prism P12.
In the construction shown in FIGS. 31 and 32, a light flux coming from the objective lens OL passes through the exit surface 111 of the first prism P11 and the entrance surface 121 of the second prism P12 and is then image-inverted and reflected once toward the object side by the roof reflecting surface 122. The reflected light flux is further totally-reflected by the entrance surface 121 of the second prism P12, so that a viewfinder image is formed on the primary image forming plane in the vicinity of the exit surface 123 of the second prism P12. A reflecting member M1 is arranged to reflect a light flux coming from the primary image forming plane to lead the reflected light flux to the eyepiece lens EL.
In the conventional viewfinder optical system shown in FIGS. 31 and 32, the exit surface 111 of the first prism P11 and the entrance surface 121 of the second prism P12 are decentered with respect to the optical axis of the objective lens OL or the eyepiece lens EL. Further, in order to cause a light flux reflected from the roof reflecting surface 122 of the second prism P12 to be totally reflected by the entrance surface 121 of the second prism P12, i.e., in order to utilize the entrance surface 121 of the second prism P12 both for transmission and reflection, the exit surface 111 of the first prism P11 and the entrance surface 121 of the second prism P12 are disposed, almost in parallel, with the minute air gap xe2x80x9cdxe2x80x9d put therebetween, as shown in FIG. 32.
Therefore, as shown in FIG. 32, rays of light indicated by solid lines have the respective angles of refraction which differ according to the positions at which the rays pass through the exit surface 111 of the first prism P11 or according to the angles of incidence of the rays on the exit surface 111 of the first prism P11. Accordingly, an optical path length possible within the minute air gap xe2x80x9cdxe2x80x9d becomes, for example, the length a1 or a2 (a1 less than a2). Therefore, astigmatism, coma, etc., become varying with right and left sides of a view field, so that it becomes difficult to observe a good viewfinder image.
In addition, as rays of light indicated by dashed lines, the surface reflection occurs between the exit surface 111 of the first prism P11 and the entrance surface 121 of the second prism P12. As indicated by the routes of those rays, a ray reflected by the entrance surface 121 of the second prism P12 is further reflected by the exit surface 111 of the first prism P11 and is then made to enter the entrance surface 121 of the second prism P12, thereby becoming a ghost of the ordinary ray indicated by a solid line, so that a double image would be formed. Therefore, the conventional viewfinder optical system has such an disadvantage as to lower the optical performance thereof.
Further, such a ghost makes the width of a double image vary, as t1 or t2 (t1 less than t2), according to the angle of incidence of a ray on the exit surface 111 of the first prism P11. For example, even if a ghost of the double image width t1 is within a permissible range because the air gap xe2x80x9cdxe2x80x9d is minute, a ghost of the double image width t2 is conspicuous. Thus, there is a problem that a difference in the double image occurs between the right and left sides of a view field.
It is an object of the invention to provide a viewfinder optical system capable of making a good viewfinder image to be observed over the whole range of a field of view when an object image formed via an objective lens unit is converted, by utilizing an image inverting unit, into a non-inverted erecting image to be observed through an eyepiece lens unit.
To attain the above object, in accordance with an aspect of the invention, there is provided a viewfinder optical system, which comprises an objective lens unit, an image inverting unit for converting an object image formed via the objective lens unit into a non-inverted erecting image, and an eyepiece lens unit for observing the non-inverted erecting image, wherein the image inverting unit comprises a first transparent body and a second transparent body which are disposed with an interval put therebetween, the second transparent body having only a function of transmitting a ray of light, and wherein the interval between the first transparent body and the second transparent body is not uniform.
According to a preferred aspect of the invention, in the viewfinder optical system, at least one of a surface of the first transparent body and a surface of the second transparent body which are opposite to each other is a rotationally-asymmetrical surface.
According to a preferred aspect of the invention, in the viewfinder optical system, the first transparent body has a surface having only a function of reflecting a ray of light, and a surface having both a function of reflecting a ray of light and a function of transmitting a ray of light.
According to a preferred aspect of the invention, in the viewfinder optical system, the second transparent body has a second entrance surface for transmitting a light flux coming from the objective lens unit, and a transmission surface disposed at an acute angle with the second entrance surface, the first transparent body consists of a first entrance surface disposed with the interval put between the transmission surface and the first entrance surface and arranged to allow a light flux coming from the transmission surface to enter the first entrance surface, a reflecting surface arranged to reflect a light flux coming from the first entrance surface toward the first entrance surface, a total-reflection surface provided at a part of the first entrance surface and arranged to totally reflect a light-flux coming from the reflecting surface, and an exit surface arranged to allow a light flux coming from the total-reflection surface to exit, and the image inverting unit further comprises a reflecting member arranged to reflect a light flux coming from the exit surface toward the eyepiece lens unit.
According to a preferred aspect of the invention, in the viewfinder optical system, the to transmission surface is a rotationally-asymmetrical surface.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.